The conventional hydraulic dockboard is mounted in a pit or depression in the loading dock and includes a ramp which is pivoted at its rear edge to the frame or supporting structure. In addition, a lip is hinged to the forward edge of the ramp and is movable between a downwardly hanging pendant position and an extended position where it forms an extension to the ramp. When in use, the extended lip will engage the bed of a truck positioned in front of the loading dock to bridge the gap between the dock and the truck. After the loading operation is completed and the truck pulls away from the dock, the lip will pivot downwardly to the pendant position, and the ramp will pivot downwardly to a stored position.
In a conventional hydraulically operated dockboard, a main hydraulic cylinder unit interconnects the frame and the ramp, and acts to pivot the ramp from a horizontal dock level position to the upwardly inclined position. In addition, a lip cylinder unit interconnects the ramp and the lip and pivots the lip from the downwardly hanging pendant position to the extended position.
U.S. Pat. Nos. 4,365,374 and 4,641,388 describe hydraulically operated dockboards in which the flow of hydraulic fluid to the lip lifting cylinder is controlled by, and supplied through, the main hydraulic cylinder unit. In accordance with U.S. Pat. No. 4,641,388, a two-way control valve is mounted in the pressure line extending between the main hydraulic cylinder and the lip lifting cylinder and the valve is also connected to a return line leading to the reservoir for the hydraulic system. Thus, the valve connects the lip lifting cylinder to either the pressure supply line or to the return line. When the main hydraulic cylinder unit is operated to raise the ramp, the piston rod of the cylinder is extended, and as the piston approaches the end of its stroke of travel, a passage is opened which supplies hydraulic fluid from the end of the main cylinder unit through the pressure line to the lip lifting cylinder to thereby operate the lip lifting cylinder and pivot the lip to the extended position.
As described in the aforementioned patent, when the flow of pressurized fluid to the main cylinder unit is terminated, the combined weight of the ramp and lip will cause the main cylinder unit to retract and the ramp will pivot downwardly until the extended lip engages the bed of a truck parked in front of the loading dock. A restricted orifice in the return line extending between the main cylinder and the reservoir will control the rate of descent of the ramp, resulting in the pressure being maintained in the main cylinder and the lip cylinder until the descent of the ramp is arrested.
When the extended lip engages the bed of the truck, the pressure in both the main cylinder unit and the lip lifting cylinder will drop to near ambient pressure and the drop in pressure will enable a biasing force to move the control valve to a position where the lip cylinder is connected to the reservoir via the return line. After the loading operation has been completed, the lip will pivot downwardly by gravity, forcing the fluid from the lip cylinder through the valve and back to the reservoir.
With the construction of U.S. Pat. No. 4,641,388 the lip will return to its downwardly hanging pendant position without the necessity of the ramp being moved to a below dock position.